On 2020/10/30 下午4:47, Michael S. Tsirkin wrote:
On Tue, Oct 27, 2020 at 12:47:34AM -0500, Mike Christie wrote:
On 10/25/20 10:51 PM, Jason Wang wrote:
On 2020/10/22 上午8:34, Mike Christie wrote:
Each vhost-scsi device will need a evt and ctl queue, but the number
of IO queues depends on whatever the user has configured in userspace.
This patch has vhost-scsi create the evt, ctl and one IO vq at device
open time. We then create the other IO vqs when userspace starts to
set them up. We still waste some mem on the vq and scsi vq structs,
but we don't waste mem on iovec related arrays and for later patches
we know which queues are used by the dev->nvqs value.
Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
---
drivers/vhost/scsi.c | 19 +++++++++++++++----
1 file changed, 15 insertions(+), 4 deletions(-)
Not familiar with SCSI. But I wonder if it could behave like vhost-net.
E.g userspace should known the number of virtqueues so it can just open
and close multiple vhost-scsi file descriptors.
One hiccup I'm hitting is that we might end up creating about 3x more vqs
than we need. The problem is that for scsi each vhost device has:
vq=0: special control vq
vq=1: event vq
vq=2 and above: SCSI CMD/IO vqs. We want to create N of these.
Today we do:
Uerspace does open(/dev/vhost-scsi)
vhost_dev_init(create 128 vqs and then later we setup and use N of
them);
Qemu does ioctl(VHOST_SET_OWNER)
vhost_dev_set_owner()
For N vqs userspace does:
// virtqueue setup related ioctls
Qemu does ioctl(VHOST_SCSI_SET_ENDPOINT)
- match LIO/target port to vhost_dev
So we could change that to:
For N IO vqs userspace does
open(/dev/vhost-scsi)
vhost_dev_init(create IO, evt, and ctl);
for N IO vqs Qemu does:
ioctl(VHOST_SET_OWNER)
vhost_dev_set_owner()
for N IO vqs Qemu does:
// virtqueue setup related ioctls
for N IO vqs Qemu does:
ioctl(VHOST_SCSI_SET_ENDPOINT)
- match LIO/target port to vhost_dev and assemble the
multiple vhost_dev device.
The problem is that we have to setup some of the evt/ctl specific parts at
open() time when vhost_dev_init does vhost_poll_init for example.
- At open time, we don't know if this vhost_dev is going to be part of a
multiple vhost_device device or a single one so we need to create at least 3
of them
- If it is a multiple device we don't know if its the first device being
created for the device or the N'th, so we don't know if the dev's vqs will
be used for IO or ctls/evts, so we have to create all 3.
When we get the first VHOST_SCSI_SET_ENDPOINT call for a new style multiple
vhost_dev device, we can use that dev's evt/ctl vqs for events/controls
requests. When we get the other VHOST_SCSI_SET_ENDPOINT calls for the
multiple vhost_dev device then those dev's evt/ctl vqs will be ignored and
we will only use their IO vqs. So we end up with a lot of extra vqs.
The issue Jason's hinting at is how can admins control the amount
of resources a given qemu instance can consume?
After all vhost vqs all live in host kernel memory ...
Limiting # of open fds would be one way to do that ...
The need to share event/control vqs between devices is a problem though,
and sending lots of ioctls on things like reset is also not that elegant.
Jason, did you have a good solution in mind?
Nope, I'm not familiar with SCSI so I don't even know sharing evt/cvq is
possible. Consider VHOST_SCSI_MAX_VQ is already 128 per device. Mike's
proposal seems to be better.
Thanks
One other question/issue I have is that qemu can open the /dev/vhost-scsi
device or it allows tools like libvirtd to open the device and pass in the
fd to use. For the latter case, would we continue to have those tools pass
in the leading fd, then have qemu do the other num_queues - 1
open(/dev/vhost-scsi) calls? Or do these apps that pass in the fd need to
know about all of the fds for some management reason?
They know about all the fds, for resource control and priveledge
separation reasons.
